In a fiber-based composite, fibers act as a reinforcing agent, while a matrix material localizes the fibers and, in some cases, controls their orientation. The matrix material also serves as a load-transfer medium between fibers within the composite. Due to their exceptional mechanical properties, carbon nanotubes (CNTs) have been used to further reinforce fiber composite materials. However, incorporation and alignment of CNTs on fibers and/or in composites has been problematic. Current methods of growing carbon nanotubes on fibers result in entangled and non-aligned carbon nanotubes with low weight percentage/concentration of carbon nanotubes. Moreover, some fiber-based substrates are sensitive to the temperatures at which CNTs are grown. This temperature sensitivity can manifest as an inherent instability of the fiber material at CNT growth temperatures. Temperature sensitivity in the CNT growth process can also be the result of CNT nanoparticle catalyst sintering due to nanoparticle mobility on the fiber surface. Improved methods for in situ growth of carbon nanotubes on different fiber-based substrates would be useful in producing greater strength composite materials as well as in other mechanical thermal and electrical applications. The present invention satisfies this need and provides related advantages as well.